YUIのフォントサイジングはIE用とかの記述あるし、
そろそろちゃんと見なおそうとおもってググった。

【スマートフォン向けサイト】フォントサイズの指定には、remを使おう。 | casemobile by hi-posi inc. :
http://case-mobile-design.com/about-css3rem/

Font sizing with rem – Snook.ca :
http://snook.ca/archives/html_and_css/font-size-with-rem

フォントサイズの指定はCSS3の「rem」が便利そう ｜ HTML5 – CSS3 mag :
http://html5-css3.jp/tips/css3-rem.html

とりあえずリセットcssを下記のように変更してrem基準を使ってみる
[css]
@charset “utf-8”;
/*
Eric Meyer’s Reset CSS v2.0 (http://meyerweb.com/eric/tools/css/reset/)
http://cssreset.com
*/
html, body, div, span, applet, object, iframe,
h1, h2, h3, h4, h5, h6, p, blockquote, pre,
a, abbr, acronym, address, big, cite, code,
del, dfn, em, img, ins, kbd, q, s, samp,
small, strike, strong, sub, sup, tt, var,
b, u, i, center,
dl, dt, dd, ol, ul, li,
fieldset, form, label, legend,
table, caption, tbody, tfoot, thead, tr, th, td,
article, aside, canvas, details, embed,
figure, figcaption, footer, header, hgroup,
menu, nav, output, ruby, section, summary,
time, mark, audio, video {
margin: 0;
padding: 0;
border: 0;
font-size: 100%;
font: inherit;
vertical-align: baseline;
}
/* HTML5 display-role reset for older browsers */
article, aside, details, figcaption, figure,
footer, header, hgroup, menu, nav, section {
display: block;
}
body {
line-height: 1;
}
ol, ul {
list-style: none;
}
blockquote, q {
quotes: none;
}
blockquote:before, blockquote:after,
q:before, q:after {
content: ”;
content: none;
}
table {
border-collapse: collapse;
border-spacing: 0;
}

/* add a little bit rules */
html {
overflow-y: scroll;
}
img,
object,
embed {
vertical-align: top;
}

input, select {
vertical-align:middle;
}

//FONT SIZING WITH REM
//http://snook.ca/archives/html_and_css/font-size-with-rem
html {
font-size: 62.5%;
}
body {
font-size: 18px; font-size: 1.8rem;
}

/* YUI 3.4.1 (build 4118) Copyright 2011 Yahoo! Inc. All rights reserved.
Licensed under the BSD License. http://yuilibrary.com/license/ */

// Nudge down to get to 13px equivalent for these form elements
select, input, button, textarea {
font:99% arial,helvetica,clean,sans-serif;
}

// To help tables remember to inherit
table {
font-size:inherit;
font:100%;
}

[/css]